Exhaust gas discharged from an internal combustion engine (for example, a diesel engine or a gasoline engine) may contain particulates such as soot. Exhaust gas containing such particulates is purified by means of collecting the particulates through use of a filter. As the need arises, the filter is heated to a high temperature so as to burn particulates accumulating on the filter to thereby remove them. Therefore, if the filter suffers breakage or a like failure, unpurified exhaust gas is discharged directly to the downstream side of the filter. Therefore, there has been demanded a particulate sensor which can detect particulates contained in exhaust gas in order to directly measure the amount of particulates contained in exhaust gas or to detect a failure of the filter.
For example, Patent Document 1 discloses a particulate measurement method and apparatus. In the method disclosed in Patent Document 1, an ionized gas containing positive ions is mixed with exhaust gas which is introduced from an exhaust pipe into a channel and which contains particulates, so as to electrify the particulates, and the particulates are then released to the exhaust pipe. A current (signal current) which flows in accordance with the amount of the released, charged particulates is detected so as to detect the particulate concentration.